Metal clad switch plants for high voltages with obturating means for preventing destructive pressure rise

ABSTRACT

The metal envelope of a gas insulated switch plant is divided into a number of detachably interconnected compartments which are separated from each other in gastight manner and insulating gas is supplied to these compartments. Some of the compartments are of such small volumes that the heat generated by a short circuit therein, for example, might cause a pressure rise of the insulating gas which is sufficient to rupture the compartment. A conduit is provided between each of these small compartments and another compartment which is itself of large enough volume, or which is cumulatively with the first compartment of large enough volume, so as to prevent such destructive pressure rise. Each such conduit is normally blocked or substantially blocked by a rupturable diaphragm which breaks, in response to the inception of a destructive pressure rise, to allow full flow communication between the two compartments so connected.

United States Patent [72] inventor Rintje Boersma I-Iarmelen, Netherlands Appl. No. 41,781 Filed June 5, 1970 Patented Nov. 9, 1971 Assignee N.V. COQ

Utrecht, Netherlands Continuation of application Ser. No. 747,313, July 24, 1968, now abandoned.

METAL CLAD SWITCH PLANTS FOR HIGH VOLTAGES WITI-I OBTURATING MEANS FOR PREVENTING DESTRUC'IIVE PRESSURE RISE Primary Examiner-Lewis H. Myers Assistant Examiner-Gerald P. Tolin ABSTRACT: The metal envelope of a gas insulated switch plant is divided into a number of detachably interconnected compartments which are separated from each other in gastight manner and insulating gas is supplied to these compartments. Some of the compartments are of such small volumes that the heat generated by a short circuit therein, for example. might cause a pressure rise of the insulating gas which is sufficient to rupture the compartment. A conduit is provided between each of these small compartments and another compartment which is itself of large enough volume, or which is cumulatively with the first compartment of large enough volume, so as to prevent such destructive pressure rise. Each such conduit is normally blocked'or substantially blocked by a rupturable diaphragm which breaks, in response to the inception of a destructive pressure rise, to allow full flow communication between the two compartments so connected.

PATENTEDunv 91911 sum 1 or 2 I INVENTOR RiNTJ E' BDERSNA ATTORNEYS PATENTEDNUV 9 I971 SHEET 2 UF 2 INVENTOR RiNTJE BO'ERSMA ATTORNI'IYS METAL CLAD SWITCH PLANTS FOR HIGH VOLTAGES WITH OBTURATING MEANS FOR PREVENTING DESTRUCTIVE PRESSURE RISE This application is a continuation of Ser. No. 747,313, filed July 24, 1968, and now abandoned.

The invention relates to a metal clad switch plant for high voltages comprising a metal envelope to be connected to earth and containing compressed gas, said envelope being divided into a number of compartments, in which at least one bus-barsystem and the devices and conductors, such as switches, terminal members for cables, current and voltage transformers for measurement and protection, excess voltage cutouts, connecting conductors, etc., belonging to a number of switching units connected to said bus-bar system are accommodated. The invention relates particularly to a switch plant of this kind for very high voltages, say voltages of 200 kv. and higher, in which the live parts are surrounded by an insulating gas of high insulating value, e.g., sulfurhexafluoride (SP Such a switch plant must be provided, for structural reasons and for the sake of safety, with a metal envelope which consists of a number of detachably interconnected compartments separated from one another in a gastight manner. Some of these compartments may be of such small volume that when a serious disturbance occurs, e.g., a full earth contact of considerable power, the pressure of the gas in such a compartment increases so rapidly that the envelope is damaged or even gives way, before the electrically conducting parts in said compartment are disconnected from the power source by the protecting devices.

An object of the invention is to provide a metal clad switch plant in which small compartments may be provided without any danger as above described, and in which care is taken that in all such compartments the gas pressure required for good insulation is maintained under all circumstances. In accordance with the invention this object is achieved by connecting each compartment with a supply conduit containing compressed gas and by providing a connecting passage having a relatively large cross-sectional area between each compartment having a volume which is smaller than a given volume and another compartment having a volume together with that of the first mentioned compartment which is bigger than said given volume, a device being provided in said passage which under circumstances of normal operation keeps said passage closed or makes it locally narrower but which under other circumstances can be brought into a condition in which said passage is fully opened. The connection between the two compartments increases the volume of the space, in which a full earth contact or another serious disturbance occurs. Due, to this increase of volume the speed with which the gas pressure increases in the combined bigger space is considerably reduced, so that the protecting devices are able to switch otf the disturbed compartment from the live parts before the gas pressure in said compartment has exceeded a dangerous value.

Since there are cases in which the connection between the two compartments must be effected with great speed, an arrangement is recommended in which the connecting passage is kept closed under circumstances of normal operation by a breaking diaphragm. When the pressure rises in the compartment of small volume, this diaphragm breaks long before the metal envelope is endangered.

As each compartment of the switch plant has to be permanently connected to a supply conduit for compressed gas said connecting passage may, at the same time, be used for this connection. The connecting passage then may, under normal circumstances of operation, be made narrower by a breaking diaphragm provided with a throttle opening.

If the connecting passage comprising the breaking diaphragm is mounted between a too small compartment and a compartment having a sufficiently big volume to prevent the occurrence of a dangerous overpressure therein, a breaking diaphragm working in one direction only may be used, that means the diaphragm may be so constructed and/or supported as to break only when the pressure of the gas contained in the compartment, of which the volume is too small, exceeds a predetermined value.

The invention may advantageously be applied to a metal clad switch plant comprising at least one switching unit provided with at least one isolating switch for the electrical interconnection of a first device accommodated in a first compartment and a bus bar of the bus-bar system or a second device accommodated in a second compartment which is separated from said first compartment, the two cooperating switching contacts of said isolating switch being contained in a switching chamber which is separated from at least one of the said two compartments and which may be divided by a movable metal partition wall into two parts containing each a switching contact of the isolating switch. If in such a switch plant the mentioned partition wall is brought into its closed position, in most cases the volume of the part of said switching chamber formed by said partition wall will become dangerously small. To reduce the danger of explosion of the said too small separated space a connecting passage having a sealing member may be provided between the compartment separated from said switching chamber and the part of said chamber confined between said compartment and the partition wall, when the latter is in its closed position, said compartment and the said part of the switching chamber being-in open communication with one another when said partition wall is in its closed position. This sealing member provided in the connecting passage may be constructed as a valve which is coupled with the partition wall and is opened when the partition wall is brought into its closed position. Under circumstances of normal operation, when the partition wall is out of action and the isolating-switch can be opened and closed, the connecting passage is closed, so that the said compartment and the switching chamber of the isolating switch are separated from one another in a gastight manner. However, it is also possible to seal the connecting passage by means of a breaking diaphragm and to provide an auxiliary conduit containing a valve between the compartment which is separated from the switching chamber of the isolating switch and the part of said chamber which is confined between the said compartment and the closed partition wall, said valve being coupled with said partition wall and being opened when the partition wall is brought into its closed position.

The invention will be further elucidated with the aid of the drawing. Therein is:

FIG. 1 a diagram of a switching unit of a switch plant according to the invention, said switching unit being adapted to be connected with either one of two bus-bar systems,

FIG. 2 a vertical sectional view of a part of such a switching unit and FIG. 3 a vertical sectional view of a modification of the part of the switching unit shown in FIG. 2.

In FIG. 1 the switching unit is used to connect a cable I to the bus-bar 2 of one bus-bar system or to the bus-bar 3 of the other bus bar system of a switch plant comprising two bus-bar systems. The cable is connected to a terminal member 4 which is connected, through a conductor adapted to be divided by plug-contact connections 5, 6 into three parts 7, 8, 9, to one end of a circuit breaker 10 comprising four series connected pairs of cooperating switching contacts. Said circuit breaker is only used to interrupt the electric circuit and it is connected in series with an isolating switch ll serving the purpose of keeping said circuit interrupted and closing same. This isolating switch is connected both to the movable switching contact 12 of a bus-bar isolating switch for the bus-bar 2 and to the movable switching contact 14 of a bus-bar isolating switch for the bus-bar 3. The fixed contacts 13 and 15 belonging to these bus-bar isolating switches and cooperating with the movable switching contacts 12 and 14 are permanently connected with the bus-bar 2 and 3. Earthing switches are designated by 16 and 17 and a capacitor for voltage measurement is indicated by 18, The cable terminal member 4 is connected through a conductor provided with plug-contact connections 19, 20 to an excess voltage cutout 21.

The switching chambers of the bus-bar isolating switches 12, 13 and l4, l confined between lead-in insulators 22, 23 and 24, 25 can be divided by movable partition walls 26 and 27 into two parts 28, 29 and 30, 31 which are separated from one another in a gastight manner.

All mentioned parts of the switching unit and the two bus bars 2 and 3 are accommodated in a metal envelope 32 filled with compressed insulating gas. Said envelope is divided into a number of compartments separated from one another in a gastight manner.

From F16. 1 it appears that the compartment 35 extending between lead-in insulators 33, 34 and containing the plug-contact connections 5, 6 and the coupling conductor 8; the compartment 38 extending between lead-in insulators 36, 37 and containing plug-contactconnections 19, 20 and the coupling conductor extending therebetween; the compartment 39 containing the excess voltage cutout 21; and the two chambers 28, respectively housing the bus-bar isolating switches 12, 13 and 14, 15, each being defined between a respective lead-in insulator 22, 24 and a closed partition wall 26, 27 have relatively small volumes. The danger of small closed spaces in a switch plant of the kind referred to is that, if a serious disturbance, e.g., a full earth contact of considerable power, occurs in such a space, the pressure increases so rapidly, due to the production of heat that the envelope runs the risk of giving way before the protection devices has been set in operation. In order to decrease this danger the compartment and the compartment 40 comprising the circuit breaker 10, the isolating switch 11, the movable switching contacts 12, 14 of the bus-bar isolating switches 12, 13 and 14, 15, the measuring capacitor 18, the earthing switch 16 and current transformers 41, 42 are interconnected by a wide connecting passage 43 provided with a breaking diaphragm 44, in which a throttle opening 45 is made. A like connecting passage 46 containing a breaking diaphragm 48 provided with a throttle opening 47 interconnects the compartments 38 and 39. Furthen'nore, wide connecting passages 49 and 50 provided with controllable valves 51 and 52 are mounted between the parts 28 and 30 of the switching chambers of the bus-bar isolating switches and the compartment 40. These valves are so coupled with the movable partition walls 26 and 27 as to be opened when these partition walls are brought into their closed position. Due to the provision of the connecting passages 49 and 50 the parts 28 and 30 of the switching chambers of the bus-bar isolating switches separated by the partition walls 26 and 27 form, so to say, an integral part of the big compartment 40.

lf the compartment 35, 38 or 39 a disturbance should occur, whereby the gas pressure will increase, the breaking diaphragm 44 or 48 will tear or break, before the metal envelope runs the risk to give way, since the compartment 35 will thereby be brought into communication with the big compartment 40 by the wide connecting passage 43 and the compartments 38 and 39 will be interconnected by the passage 46, so that the volumes of the spaces, in which the disturbance occurs, will be sufficiently increased to prevent the explosion of the metal envelope.

In order to maintain, under all circumstances, the gas pressure in the various compartments supply conduits 53, 54, 55 for the supply of insulating gas, e.g., sulfurhexafluoride (SF under pressure of say 3.5 mm, are provided which lead the gas into the bus-bar compartments 56, 57, the compartments 39 and 40 and the compartment 58 containing the cable terminal member 4. This gas is led through the throttle openings 45 and 47 from the compartments 40 and 39 to the compartments 35 and 38. The compartments 59 containing the earthing switch 17 communicates for its gas supply through an opening 60 with the compartment 58 and for the same purpose the switching chambers 28, 29 and 30, 31 of the bus-bar isolating switches 12, 13 and l4, 15 are connected through openings 61 and 62 made in the insulators 23 and 25 with the bus-bar compartments 56, 5'7. lf the partition walls 26 and 27 are closed the parts 28 and 30 of these switching chambers are connected through the connecting passages 49 and 50 opened by the valves 51 and 52 with the compartment 40, so that they communicate with the supply conduit 55.

FIG. 2 shows a part of a switch plant illustrated in FIG. 1 comprising a bus-bar compartment 63 containing a bus-bar 64, the fixed contact 65 of a bus-bar isolating switch connected to said bus bar, the movable switching contact 66 of said isolating switch, said switching contact being in its open position, the partition wall 69 which divides the switching chamber of the bus-bar isolating switch into two parts 67 and 68 and a portion of the compartment 70, containing the circuit breaker. In this embodiment the part 67 of said switching chamber confined between the lead-in insulator 71 and the closed partition wall 69 is connected with the compartment 70 both by a wide connecting passage 72 provided with an uninterrupted breaking diaphragm 73 and by a narrow conduit 74 provided with a valve 75. This valve is so coupled with the partition wall 69 as to be opened when the partition wall is brought into its closed position. The partition wall 69 illustrated includes top and bottom plates 100 and 101 which are urged apart by means of biasing springs 102 and which are joined by pairs of links 103 and 104 to a reciprocable actuator member 105. The member 105 is reciprocated by suitable means (not shown) to position the partition wall 69 in closed position as shown in FIG. 2 or to move it to open position in v which it is disposed within the chamber 106 within the housing 107. The valve 75 is coupled to the actuating mechanism for the partition wall so that the valve is open when the partition wall is in closed position. Any suitable means may be employed to so couple the valve 75 to the actuating mechanism for the partition wall, the coupling being functionally indicated by the dashed line 108 in FIG. 2. The narrow conduit 74 has the purpose to maintain the gas pressure in the part 67 of the switching chamber, when the partition wall is closed, and the wide passage 72 is opened by the tearing or breaking of the diaphragm 73, when the pressure of the gas in said part 67 of the switching chamber exceeds, due to a disturbance, a given value.

ln the modification shown in FIG. 3 of the embodiment illustrated in FIG. 2 the connecting passage 72 with its breaking diaphragm 73 and the narrow conduit 74 with its valve 75 are replaced by a sleeve 76 provided with ports 77. This sleeve is loaded by springs 78 and is mounted for sliding in a ring 79 which has corresponding ports and supports the lead-in insulator 71. If the partition wall 69 is inactive the sleeve 76 is forced by the springs 78 into a position, in which the ports 77 and 80 lie in ofiset relation, so that they are closed. However,

if the partition wall 69 is closed, it forces the sleeve 76 into a position (see FIG. 3), in which the ports 77 and 80 correspond with each other, so that the part 67 separated by the partition wall from the switching chamber of the bus-bar isolating switch 65, 66 is in open communication with the big compartment 70.

I claim:

1. A switch plant for high voltage electrical systems comprising, in combination;

a closed metal envelope adapted to be connected to earth;

means dividing said envelope into a number of compartments separated from each in gastight manner;

means for supplying pressurized insulating gas to said compartments;

a high-voltage electrical system enclosed within said envelope and including a bus bar in one of said compart ments, a cable in another of said compartments and which is accessible for external electrical connections, circuit breaker means in a further compartment and connected to said cable, and a bus bar isolating switch selectively connecting said circuit breaker to said bus bar;

said compartments being of difierent volumes which are smaller and greater than a predetennined volume which is sufficient to prevent destructive pressure rise of said insulating gas in the event of a serious electrical disturbance,

passage means connecting each compartment having a volume less than said predetermined volume with another compartment having a volume with which it additively is at least equal to said predetermined volume, said passage means being of a cross-sectional area sufiicient to prevent said destructive pressure rise of said insulating gas in the event of a serious electrical disturbance in said compartment having a volume less than said predetermined volume; and

obturating means in said passage means for minimizing the cross-sectional area of said passage means during ordina' ry operation of the switch plant and fully to open said passage means at the occurrence of extraordinary conditions.

2. A switch plant as defined in claim 1, wherein said obturating means is in the form of a rupturable diaphragm.

3. The switch plant as defined in claim 2, wherein said diaphragm is provided with a relatively small aperture.

4. A switch plant for handling high voltages comprising, in combination;

a closed metal envelope adapted to be electrically grounded and including at least a pair of conduit portions, a housing portion, and spaced pairs of closure means, each pair of closure means defining a secondary compartment therebetween separating the interior of said housing portion from the interior of a corresponding conduit portion and thereby defining separate compartments in said housing and conduit portions, said secondary compartments being of volumes insufficient to prevent destructive pres sure rise of insulating gas therein due to serious electrical disturbance in such secondary compartments whereas that compartment defined by said housing is of sufficient volume to prevent such destructive pressure rise;

a bus bar in one of said conduit portions and a cable in the other of said conduit portions;

circuit breaker means in said compartment defined by said housing;

a switch in that secondary compartment between said one conduit portion and said housing portion and including separable contacts;

conductors connecting the separable contacts of said switch respectively to said bus bar and to said circuit breaker means;

conductor means connecting said circuit breaker means to said cable and extending through that secondary compartment between said housing portion and said other conduit portion;

means supplying pressurized insulating gas to said separate compartments defined by said housing and conduit portions for maintaining substantially equal insulating gas pressure therein; and

bypass means connecting each secondary compartment to said compartment defined by said housing for allowing substantially unrestricted flow of insulating gas from each secondary compartment to said compartment defined by said housing in response to sudden pressure increase in such secondary compartment which may be due to a malfunction within such secondary compartment.

5. A switch plant as defined in claim 4, wherein one closure means of that pair thereof separating said housing from said one conduit portion is movable between operative and inoperative positions, and the bypass means associated therewith includes a conduit and a valve operable between closed and open positions in response to movement of said one closure means between its inoperative and operative positions.

6. A switch plant as defined in claim 5, wherein the bypass means associated with that secondary compartment separating said other conduit portion from said housing is in the form of a conduit having a rupturable diaphragm therein.

7. A switch plant as defined in claim 6, wherein said rupturable diaphra is provided with a relatively small aperture. 8. A switch p ant as defined in claim 4, wherein the bypass means associated with that secondary compartment separating said other conduit portion from said housing is in the form of a conduit having a rupturable diaphragm therein.

9. A switch plant as defined in claim 8, wherein said rupturable diaphragm is provided with a relatively small aperture.

10. A metal clad switch plant for high voltages comprising a closed metal envelope intended for connection to earth; said envelope being divided into a number of individual compartments having volumes which are smaller and bigger than a predetermined volume V, respectively, in which said volume V is just sufficient to prevent destructive pressure rise of insulating gas due to a serious electrical disturbance within a compartment; switches, terminal members for the connection of cables to the switch plant, devices for measurement and protection, and connecting conductors; said switches, terminal members, devices and conductors being insulatedly mounted in compartments of the envelope and constituting feed and delivery circuits of the switch plant; bus bars insulatedly mounted in compartments of the envelope and interconnecting said circuits; conduits for the supply of compressed insulating gas; each compartment of the envelope being in open communication with such a gas supply conduit; relatively wide connecting passages and obturating devices provided in said passages; each one of said passages extending between a compartment, of which the volume is smaller than the predetermined volume V, and another compartment, of which the volume makes together with that of the former compartment a volume which is at least equal to the volume V; and each obturating device being adapted to keep, at ordinary operation of the switch plant, the passage area of the relative connecting passage at a minimum and to open said passage to its full extent at the occurrence of extraordinary conditions.

11. A metal clad switch plant as claimed in claim 10, in which the obturating device is a rupturable diaphragm.

12. A metal clad switch plant as claimed in claim 11, in which the obturating device is a rupturable diaphragm provided with a relatively small aperture.

13. A metal clad switch plant as claimed in claim 10, including isolator switches forming part of the feed and delivery circuits of the switch plant, each isolator switch including a fixed contact permanently connected to one part of the relative circuit and a movable contact permanently connected to another part of said circuit; each isolator switch being accommodated in a compartment which is separated from at least one other compartment; movable partition walls which are each adapted to be moved into and out of an isolator switch compartment and to divide the relative isolator switch compartment into two chambers, when the relative isolator switch is in its open condition; the chamber confined between said other compartment and the partition wall being connected by a connecting passage provided with an obturating device to said other compartment and coupling means provided between said partition wall and said obturating device; said coupling means being adapted to close said obturating device, when the partition wall is moved out of the isolator switch compartment, and to open said obturating device when the partition wall is moved into the isolator switch compartment.

14. A metal clad switch plant as claimed in claim 13, in which a second connecting passage is provided between the chamber forming part of the isolator switch compartment and confined between said other compartment and said partition wall on one hand and said other compartment on the other hand; and a rupturable diaphragm in said second connecting passage. 

1. A switch plant for high voltage electrical systems comprising, in combination; a closed metal envelope adapted to be connected to earth; means dividing said envelope into a number of compartments separated from each in gastight manner; means for supplying pressurized insulating gas to said compartments; a high-voltage electrical system enclosed within said envelope and including a bus bar in one of said compartments, a cable in another of said compartments and which is accessible for external electrical connections, circuit breaker means in a further compartment and connected to said cable, and a bus bar isolating switch selectively connecting said circuit breaker to said bus bar; said compartments being of different volumes which are smaller and greater than a predetermined volume which is sufficient to prevent destructive pressure rise of said insulating gas in the event of a serious electrical disturbance, passage means connecting each compartment having a volume less than said predetermined volume with another compartment having a volume with which it additively is at least equal to said predetermined volume, said passage means being of a crosssectional area sufficient to prevent said destructive pressure rise of said insulating gas in the event of a serious electrical disturbance in said compartment having a volume less than said predetermined volume; and obturating means in said passage means for minimizing the crosssectional area of said passage means during ordinary operation of the switch plant and fully to open said passage means at the occurrence of extraordinary conditions.
 2. A switch plant as defined in claim 1, wherein said obturating means is in the form of a rupturable diaphragm.
 3. The switch plant as defined in claim 2, wherein said diaphragm is provided with a relatively small aperture.
 4. A switch plant for handling high voltages comprising, in combination; a closed metal envelope adapted to be electrically grounded and including at least a pair of conduit portions, a housing portion, and spaced pairs of closure means, each pair of closure means defining a secondary compartment therebetween separating the interior of said housing portion from the interior of a corresponding conduit portion and thereby defining separate compartments in said housing and conduit portions, said secondary compartments being of volumes insufficient to prevent destructive pressure rise of insulating gas therein due to serious electrical disturbance in such secondary compartments whereas that compartment defined by said housing is of sufficient volume to prevent such destructive pressure rise; a bus bar in one of said conduit portions and a cable in the other of said conduit portions; circuit breaker means in said compartment defined by said housing; a switch in that secondary compartment between said one conduit portion and said housing portIon and including separable contacts; conductors connecting the separable contacts of said switch respectively to said bus bar and to said circuit breaker means; conductor means connecting said circuit breaker means to said cable and extending through that secondary compartment between said housing portion and said other conduit portion; means supplying pressurized insulating gas to said separate compartments defined by said housing and conduit portions for maintaining substantially equal insulating gas pressure therein; and bypass means connecting each secondary compartment to said compartment defined by said housing for allowing substantially unrestricted flow of insulating gas from each secondary compartment to said compartment defined by said housing in response to sudden pressure increase in such secondary compartment which may be due to a malfunction within such secondary compartment.
 5. A switch plant as defined in claim 4, wherein one closure means of that pair thereof separating said housing from said one conduit portion is movable between operative and inoperative positions, and the bypass means associated therewith includes a conduit and a valve operable between closed and open positions in response to movement of said one closure means between its inoperative and operative positions.
 6. A switch plant as defined in claim 5, wherein the bypass means associated with that secondary compartment separating said other conduit portion from said housing is in the form of a conduit having a rupturable diaphragm therein.
 7. A switch plant as defined in claim 6, wherein said rupturable diaphragm is provided with a relatively small aperture.
 8. A switch plant as defined in claim 4, wherein the bypass means associated with that secondary compartment separating said other conduit portion from said housing is in the form of a conduit having a rupturable diaphragm therein.
 9. A switch plant as defined in claim 8, wherein said rupturable diaphragm is provided with a relatively small aperture.
 10. A metal clad switch plant for high voltages comprising a closed metal envelope intended for connection to earth; said envelope being divided into a number of individual compartments having volumes which are smaller and bigger than a predetermined volume V, respectively, in which said volume V is just sufficient to prevent destructive pressure rise of insulating gas due to a serious electrical disturbance within a compartment; switches, terminal members for the connection of cables to the switch plant, devices for measurement and protection, and connecting conductors; said switches, terminal members, devices and conductors being insulatedly mounted in compartments of the envelope and constituting feed and delivery circuits of the switch plant; bus bars insulatedly mounted in compartments of the envelope and interconnecting said circuits; conduits for the supply of compressed insulating gas; each compartment of the envelope being in open communication with such a gas supply conduit; relatively wide connecting passages and obturating devices provided in said passages; each one of said passages extending between a compartment, of which the volume is smaller than the predetermined volume V, and another compartment, of which the volume makes together with that of the former compartment a volume which is at least equal to the volume V; and each obturating device being adapted to keep, at ordinary operation of the switch plant, the passage area of the relative connecting passage at a minimum and to open said passage to its full extent at the occurrence of extraordinary conditions.
 11. A metal clad switch plant as claimed in claim 10, in which the obturating device is a rupturable diaphragm.
 12. A metal clad switch plant as claimed in claim 11, in which the obturating device is a rupturable diaphragm provided with a relatively small aperture.
 13. A metal clad switch plant as claimed in claim 10, including isolator switches forming part of the feed and delivery circuits of the switch plant, each isolator switch including a fixed contact permanently connected to one part of the relative circuit and a movable contact permanently connected to another part of said circuit; each isolator switch being accommodated in a compartment which is separated from at least one other compartment; movable partition walls which are each adapted to be moved into and out of an isolator switch compartment and to divide the relative isolator switch compartment into two chambers, when the relative isolator switch is in its open condition; the chamber confined between said other compartment and the partition wall being connected by a connecting passage provided with an obturating device to said other compartment and coupling means provided between said partition wall and said obturating device; said coupling means being adapted to close said obturating device, when the partition wall is moved out of the isolator switch compartment, and to open said obturating device when the partition wall is moved into the isolator switch compartment.
 14. A metal clad switch plant as claimed in claim 13, in which a second connecting passage is provided between the chamber forming part of the isolator switch compartment and confined between said other compartment and said partition wall on one hand and said other compartment on the other hand; and a rupturable diaphragm in said second connecting passage. 